Bass Traps: Simple Question

[Jack Russell]

Well, I'm a bit bleary-eyed and pale-skinned having spent weeks inside the Dragon Cave, so let me shake off the cobwebs and post a real technical question.....

Much has been written about using bass traps in a mixing room. Can someone please elaborate on the why of using them? I know that you should place them in the corners to absorb the low frequencies. But, if you've sucked up most of the low end, the wouldn't your mixes end up sounding too bassy, because you would have increased the low end to compensate for what you've lost in your bass traps?

 

[scottboyher]

Well, I'm a bit bleary-eyed and pale-skinned having spent weeks inside the Dragon Cave, so let me shake off the cobwebs and post a real technical question.....

Much has been written about using bass traps in a mixing room. Can someone please elaborate on the why of using them? I know that you should place them in the corners to absorb the low frequencies. But, if you've sucked up most of the low end, the wouldn't your mixes end up sounding too bassy, because you would have increased the low end to compensate for what you've lost in your bass traps?

I am not qualified to answer this question so I will. I think it is in an attempt to stop the low frequency reflections... Not to suck up the bass frequecies from the room..

As with any frequency I think think the idea is to stop the reflections from going out of control in the room. Of course some reflection i good.



Of course I know nothing of acoustics....

 

[mshilarious]

Much has been written about using bass traps in a mixing room. Can someone please elaborate on the why of using them? I know that you should place them in the corners to absorb the low frequencies. But, if you've sucked up most of the low end, the wouldn't your mixes end up sounding too bassy, because you would have increased the low end to compensate for what you've lost in your bass traps?

No, because you still hear the direct bass, which is the idea. Bass reflections in a room can enhance or attenuate bass response, and the effect can vary at different frequencies. Without bass traps, you have no idea what is going on in your bass mix.

 

[Jack Russell]

Thanks, guys. Those are good answers.

So, if you can create a reflection free zone at your mixing point, which is what I'm attempting to do with the home studio I'm building, then what you get from the moniters is what you mix. Makes sense.

With the bass traps, then you would also need to use traps for all the other frequencies, to make sure you have a balance between the highs and lows?

But the contradiction comes when one approaches having a dead room, right? You want a controlled room, not a dead one.

Not easy to understand.

 

[pdadda]

But what about the really low end? Bass frequencies have very long wavelengths, and you will probably not be able to hear them all directly if you monitor with near fields.

 

[Jack Russell]

But what about the really low end? Bass frequencies have very long wavelengths, and you will probably not be able to hear them all directly if you monitor with near fields.

Yeah. I dunno. Plus, doesn't the length of the mixing room also impact whether you will hear the lowest of the lows?

 

[leddy]

Remember that as the sound waves pass through one another as they are reflecting around the room, certain frequencies will cancel each other out while others will be exaggerated. If you have a lot of bass frequencies cancelling out, bass traps will actually INCREASE the amount of bass you hear in the room, but it will do so in a good way - making what you hear a better representation of what's in the mix. You need to get a handle on what problems your room has before you jump in to "correcting" it.

There's loads of info around here on that. I started with using a sine wave generator and playing different frequencies up and down listening for what was louder or softer. I made some foam wedge and bass trap adjustments to the room and repeated the process until it was acceptable. It also helped me understand where my monitors begin to roll off, and helped me "learn" them better. I tried corrective EQ, putting a 31 band EQ before the power amp to the monitors and "flattening" the room that way. The experienced folks here will say that's a mistake. Having tried it, I'll agree 100%. If you were going to do it, it could only be done with a very transparant parametric. If you were going to spend the money on such an EQ, you'd be better off spending it on better monitors and room treatment.

 

[Jack Russell]

Remember that as the sound waves pass through one another as they are reflecting around the room, certain frequencies will cancel each other out while others will be exaggerated. If you have a lot of bass frequencies cancelling out, bass traps will actually INCREASE the amount of bass you hear in the room, but it will do so in a good way - making what you hear a better representation of what's in the mix. You need to get a handle on what problems your room has before you jump in to "correcting" it.

There's loads of info around here on that. I started with using a sine wave generator and playing different frequencies up and down listening for what was louder or softer. I made some foam wedge and bass trap adjustments to the room and repeated the process until it was acceptable. It also helped me understand where my monitors begin to roll off, and helped me "learn" them better. I tried corrective EQ, putting a 31 band EQ before the power amp to the monitors and "flattening" the room that way. The experienced folks here will say that's a mistake. Having tried it, I'll agree 100%. If you were going to do it, it could only be done with a very transparant parametric. If you were going to spend the money on such an EQ, you'd be better off spending it on better monitors and room treatment.

That's interesting about the 31-band e.q. That is where I'm coming from, which might show my lack of knowledge. I had always thought that you'd test the room for what frequencies are high in the range, then invert that on your 31-band e.q. between the power amp and the monitors to equal out the imperfections.

Is there a device that can read the levels when you test? I don't completely trust my ears, as I have bit of a hearing loss in one ear.*

[*Which might raise the question, "How can you mix music if you have a hearing loss?" The answer: because you want to. ]

 

[AlexW]

Doing the sine wave sweep as discussed should work. You'll be able to hear the peaks and valleys well enough; they're fairly dramatic. The low frequencies are proportionally longer and more powerful then the highs, so typically small mixing/tracking rooms need more low frequency attenuation than high. If you're building your room, just make sure you've got a symetrical space around your mix position and then treat it out from there.

 

[corban]

But what about the really low end? Bass frequencies have very long wavelengths, and you will probably not be able to hear them all directly if you monitor with near fields.

This is true for the near-fields, in which case somebody who really wants to check those low lows can get a sub. This doesn't change the need for bass traps to give your room an accurate bass representation.

(btw, I'm all talk, I don't have proper bass traps - but I'm working on it)

 

[mshilarious]

But what about the really low end? Bass frequencies have very long wavelengths, and you will probably not be able to hear them all directly if you monitor with near fields.

Sound waves don't need distance to form. I have a sub with a lowpass filter set at 200Hz, that's a wavelength of about 5.5 ft. By that theory, if I put my ear 2" (equivalent frequency wavelength: 6kHz) from the cone, I shouldn't be able to hear any direct sound. Thus the volume should not change as I move away from the speaker, since all I am hearing is reflected sound.

I assure you that is not the case, especially since I have a very dead room with ample bass trapping, so there isn't much reflected sound. It's a lot louder with my ear 2" away.

Or consider headphones, where there is no reflected sound: Can you hear frequencies below 6kHz in headphones?

The reason why people have observed that bass frequencies need distance to be heard is exactly what we are talking about: in small spaces, cancellation can reduce the amplitude of bass frequencies. Larger spaces have fewer problems with cancellation, so by moving farther away (which requires a large space), there is less cancellation.

If you don't have a sufficiently large space, then you eliminate the cancellation by eliminating the reflections.

 

[mshilarious]

This is true for the near-fields, in which case somebody who really wants to check those low lows can get a sub. This doesn't change the need for bass traps to give your room an accurate bass representation.

That's more of a function of the drivers in the nearfields--if you're using 6" drivers, they will have very substantial rolloff below 100Hz.

 

[Ethan Winer]

Jack,

> Much has been written about using bass traps in a mixing room. Can someone please elaborate on the why of using them? <

You got a lot of good advice, and I'll add one more big feature of bass traps: Besides helping to flatten the low frequency response in a room, they also reduce modal ringing and broaden the bandwidth of the peaks. There's a lot about this on my company's web site, including a video that explains resonance and ringing and the two ETF (room analysis software) graphs below. The graphs show how all three factors - LF response, ringing, and modal bandwidth - are improved by adding bass traps.

--Ethan

 

[Jack Russell]

Thanks very much everyone!

 

[leddy]

That's interesting about the 31-band e.q. That is where I'm coming from, which might show my lack of knowledge. I had always thought that you'd test the room for what frequencies are high in the range, then invert that on your 31-band e.q. between the power amp and the monitors to equal out the imperfections.

Is there a device that can read the levels when you test? I don't completely trust my ears, as I have bit of a hearing loss in one ear.*

[*Which might raise the question, "How can you mix music if you have a hearing loss?" The answer: because you want to. ]

You're correct about the EQ. That's how I did it. The problem with a graphic EQ, (as was pointed out to me by folks here) is you can't control the cutoff point between frequency bands. You could very well add as many problems as you solve. In addition, you are adding another piece of equipment to the monitoring chain which can color sound.

Now, that said, when I get close to finishing a mix, I run it through a small Mackie mixer and adjust the bass and treble up and down to kind of simulate less-than-flat listening devices. One thing I notice about really well mixed material is that adjusting the simple bass and treble controls don't affect the balance of the mix. The bass knob will just make the whole thing a little fatter or thinner, for example. If my mix has problems, adjusting the bass knob up and down will usually produce some obnoxious, boomy or muddy results or suck all the low end out of the mix. Just a personal observation / personal technique. YMMV

 

[boingoman]

That's interesting about the 31-band e.q. That is where I'm coming from, which might show my lack of knowledge. I had always thought that you'd test the room for what frequencies are high in the range, then invert that on your 31-band e.q. between the power amp and the monitors to equal out the imperfections.

]

The big problem with using any kind of EQ, graphic or parametric, is that speakers can be EQ'ed and rooms can't. No kind of EQ will make up for a bad room or placement that is cancelling all the 150hz, for instance. And eqing speaker response while the room is a factor will fuck up your mix.

You try to take the room out of the equation as much as possible, so as to just hear the monitors.

Also, as leddy said, EQ's can affect the sound, as well. People seem to recommend "learning" your monitors, as in getting used to how they sound.

Most of my work has been in live situations, where the EQ is a common tool, and you have to eq the system to the room.

This is something I did:

I set up my PA outside, away from reflections (on a big lawn), and used some measurement tools (pink noise, a 1/6 octave analyzer and a computer program with a more sophisticated analyzer) to see the response of my system.
I then used an EQ to flatten the system response. Now when I take it into a room, I know that any problems are from the room, not my gear. Then it's pretty much up to first, proper placement, and second, eqing the system to compensate for the room.

So even though people say not to use an EQ on studio monitors (I don't use one on mine), comparing the monitor's response away from reflections to it's response in your room will still give you a baseline to see what your room is doing.